In digital communications systems, a carrier signal is modulated with the digital data to be transmitted over the channel, where it typically suffers various forms of distortion, such as additive noise. The digital data is often transmitted in bursts wherein each burst consists of a number of data bits. Upon reception, the signal must be demodulated in order to recover the transmitted data.
It is common for receivers to employ direct conversion (i.e. homodyne receiver) to perform the demodulation of the received signal. The received signal is mixed with a local oscillator signal at the carrier frequency to produce I (in-phase) and Q (quadrature) baseband signals. An advantage of direct conversion receivers is that they are efficient in terms of cost and current consumption. The advantage is derived from having the incoming RF signal directly downconverted to baseband, in both I and Q components, without use of any IF frequencies.
In other receivers, the incoming RF signal is mixed down first to an intermediate frequency (IF) signal and subsequently to baseband. The IF frequency may be any convenient frequency. For example, in a Bluetooth receiver, the front-end may output a low frequency IF signal (e.g., Near-Zero IF) as low as half the bandwidth of the signal (i.e. 0.5 MHz in this case).
Considering Gaussian FSK (GFSK) modulation and considering the presence of frequency offsets, the zero-IF I and Q signals can be expressed mathematically by the following.IZIF=A cos [Φ(t)+ΔωIFt+θn]QZIF=A sin [Φ(t)+ΔωIFt+θn]  (1)where A is a constant, ΔωIF represents the frequency offset, Φ(t) represents the phase shift created by the modulating data and θn represents the contribution of random noise to the phase. Note that it is assumed there is no gain or phase mismatch. In Bluetooth low-IF systems, ωIF is usually 500 kHz and the local oscillator frequency used for downconversion from RF to IF is given by LO=ωC−ωIF where ωc denotes the carrier frequency. The downconverted I signal is expressed mathematically as follows.I=A cos(ωIFt+Φ(t)+θ)  (2)After downconversion from IF to zero-IF, the output signal is given byI=A cos(Φ(t)+θ)  (3)Differential detection of this signal calculates(Φ(t)+θ(t))−(Φ(t−T)+θ(t−T))=>ΔΦ  (4)where T represents the symbol time. In Bluetooth systems, the symbol time T is 1 microsecond. The result of differential detection yields sin(ΔΦ), which for small values of φ can be approximated as simply ΔΦ.
Numerous prior art analog techniques are available to perform the demodulation required to generate accurate output data. The modem trend, however, is to provide single chip solutions to communication applications. This requires all digital realization of all or most of the receiver circuitry in the radio. Digital realization of the radio for inclusion in single chip implementations is desirable because it offers a high performance solution at low current consumption and low gate count and hence reduced size and cost. These benefits are driving the current trend to realize as much of the radio digitally for placement on a single chip.
Any digital demodulator implementation, in particular digital demodulation of GFSK, must be able to perform in the presence of additive white Gaussian noise (AWGN), interference and frequency offsets. Frequency offsets are another form of distortion of the received signal, since they are random and must be resolved adequately in the receiver to minimize the performance degradation they cause. Considering a communication system constructed to receive GFSK in accordance with the Bluetooth standard, the receiver must be constructed to deal with frequency offsets in order to generate a reliable output signal. There exist several sources of frequency offset errors in a Bluetooth communication system as highlighted below.
First, the Bluetooth specification permits a frequency error of up to 75 kHz in the transmitted signal. Second, the Bluetooth specification also permits up to 20 ppm of frequency inaccuracy in the receiver crystal reference, which could result in up to 50 kHz of frequency offset in the receiver's local oscillator which is used in the downconversion. Further, an additional frequency offset of up to 40 kHz is allowed in the transmitted signal during transmission of long packets. Lastly, an additional 15 kHz of frequency offset may be due to clock jitter caused by using clocks derived from dividing the master local clock signal, wherein the local clock signal is hopping from frequency to frequency resulting in jitter. This frequency offset could be avoided by using more accurate clocks without clock division. Note that the first two frequency offsets are constant in nature and due to the open loop configuration of the transmitter permitted under the Bluetooth standard.
Thus, an IF signal at the receiver's demodulator input may have a total of up to 180 kHz in frequency offset. Considering a nominal frequency deviation of +/−160 kHz for a modulation index of h=0.32, in accordance with the Bluetooth specification, a possible frequency offset of 180 kHz makes reception virtually impossible. Note that using a lower modulation index of 0.28, which the Bluetooth specification allows, makes the problem even worse.
Thus, there is a need for a digital demodulator, suitable for single chip implementations, that meets the requirements of the Bluetooth specification and that overcomes the problems and disadvantages of the prior art.